Recently, William B. Mullen III disclosed in U.S. Pat. No. 5,241,133 a pad grid array plastic package. This package includes a resinous circuit carrying substrate having an array of contact pads on the bottom side and an IC unit electrically and mechanically mounted on the top side of the substrate. A protective plastic cover is transfer molded about the IC unit, covering most of the top side of the substrate. Solder balls are formed on the contact pads to provide a ball grid array that is to be soldered to a main circuit board.
However, this process utilizes transfer molding which, while widely used, has certain drawbacks. In transfer molding, the plastic material is forced under pressure through gates into a metal mold having recesses defining the shape of the cover to be produced. One of the main drawbacks of transfer molding is cost of and time delay in manufacturing metal molds. This becomes especially disadvantageous when there is a need for preparing molds for different sizes of packages or for accommodating a number of mold units in a single transfer scheme. There is also a possibility of damage caused by the liquefied thermoplastic material, to the wires electrically connecting the IC unit to the metallizations on the top side of the substrate. Another possibility is substrate deformation due to the shrinking of the plastic material in the process of curing.
Clearly, a need exists for a ball grid array plastic package which is simple to construct and produce on a mass production basis and which can be in-house assembled with the IC unit for fast prototyping.